The present invention relates to a CCD type color solid-state imager wherein photoelectric conversion elements for producing a plurality of color signals, and scanners (hereinbelow, termed "CCD shift registers") constructed of charge coupled devices (hereinbelow, termed "CCDs") and for taking out optical information from the respective elements are integrated on a semiconductor substrate.
A solid-state imager requires an imaging plate which has a resolving power comparable to that of an imaging electron tube used in the current television broadcast. This necessitates about 500.times.500 photoelectric conversion elements constituting a matrix of picture elements, and scanning elements corresponding thereto. Accordingly, the solid-state imager is fabricated by the use of the MOS large-scale integrated circuit technology with which high density of packaging is comparatively easy. In general, a CCD type imaging device (photodiode+CCD shift register), a MOS type imaging device (photodiode+MOS shift register), or the like is used as the constituent elements. FIG. 1 shows the fundamental arrangement of a CCD type solid-state imager which is featured by low noise. Numeral 1 designates a photoelectric conversion element which is made of, for example, a photodiode. Numerals 2 and 3 designate a vertical CCD shift register and a horizontal CCD shift register which serve to read out photo signals, stored in the photoelectric conversion elements, to an output end.
In a case where a color television camera is constructed using the solid-state imager in FIG. 1, three imaging plates are usually needed for converting light rays of red, green and blue into electric signals respectively. The color camera employing the three solid-state imaging plates, however, requires a color separating optical system for separating a subject into the three primary colors of red, green and blue, a special imaging lens, etc., which form serious obstacles to rendering the camera small in size and low in price. In this regard, there has been proposed a method wherein, as shown in FIG. 2 by way of example, respective photoelectric conversion elements which constitute an imaging picture element matrix are held in correspondence with color filters R, G and B which are arrayed in a checkerboard pattern and which transmit only light rays of red (R), green (G) and blue (B) respectively, whereby signals of the three primary colors are derived from a single imaging plate (refer to the official gazette of Japanese Laid-open Patent Application No. 51-112228). The color filter assembly of this arrangement has the green filters arrayed in the horizontal and vertical directions of the imaging plate in a manner to fill up the interstices of the color filters R and B. Therefore, even when the number of picture elements of the solid-state imaging plate is small, a color solid-state imager whose resolution is little deteriorated can be obtained, and the arrangement is very excellent as a system for deriving the color signals from the single imaging plate. Further, this color filter system has already been adopted in a MOS type solid-state imager, and a color solid-state imager with a single plate has been reported (N. Koike et al., "An NPN Structure 484.times.384 MOS Imager for a Single-Chip Color Camera", 1979 ISSCC Digest Tech Papers, 192).
If the color filter system stated above can be adopted for the CCD type solid-state imager having the advantage of low noise, a color solid-state imager of very high sensitivity will be realizable. However, in a case where this color filter system is applied to the CCD type imager shown in FIG. 1, the following inconveniences ascribable to the operation and construction of the CCD type device occur unlike the aforementioned case of the MOS type (FIG. 3):
(1) The interlaced scanning is carried out in the vertical direction 4. In this imager, by way of example, the signals of picture elements in odd-numbered rows (1, 3, 5, . . . , 2N-1) are read out in the first field, and those of picture elements in even-numbered rows (2, 4, 6, . . . , 2N) are read out in the second field. As a result, in the first field of the next frame, the signals of rows not read out in the immediately preceding field (that is, odd-numbered rows) are read out in superposition on new signals (this phenomenon is usually called "image lag"). Since the solid-state imager has a high switching speed, one of its important features is that the image lag does not develop. In actuality, however, the image leg attributed to the interlaced read-out system as above described develops.
(2) In the vertical direction, the filters of the same color are disposed over two rows of picture elements 5. Therefore, in spite of employing the interlaced scanning and the checkerboard-like color filters, only a resolution which corresponds to half of the number of picture elements can be attained in the vertical direction. In consequence, the picture quality degrades. Along with the image lag explained in Item (1), this forms a serious cause for hindering the solid-state imager from being put into practical use.
As methods for improving these problems, regarding the image lag described in Item (1), it has been known to prevent the occurrence of the image lag by the interlaced scanning system which selects two rows of picture elements at the same time (refer to the official gazette of Japanese Laid-open Patent Application No. 51-57123). Regarding the resolution described in Item (2), it has been known that the degradation of the resolution can be prevented by a method in which the filter of one color is held in correspondence with a single picture element also in the vertical direction and in which two rows of picture elements are simultaneously read out through a plurality of signal output lines. Accordingly, the conceptual measures on the improvements have already been proposed. It is actually impossible, however, to simply apply these measures to the CCD type solid-state imager.
The reasons will be listed below:
(1) The MOS type is of the dot address (X.multidot.Y address) system as to the read-out of signals, and can derive signal charges in any direction. In contrast, the CCD type is of the system in which signal charges are unidirectionally transferred, and actually it cannot adopt the interlaced system of the simultaneous two rows in view of the operating principle.
(2) Also in the CCD type imager, is not impossible to dispose a plurality of CCD registers for taking out signals. Since, however, the rate of aperture for incident light is low (the sensitivity is low) even in case of the single register, there occurs the side effect that the area of picture elements becomes still smaller due to the plurality of registers and that a sensitivity required in practical use cannot be attained.
As understood from the above explanation, how the foregoing two measures of improvements tentatively realized in the MOS type imager are realized in concrete forms is deemed an important problem to-be-solved of the CCD type imager having the advantage of low noise.